Optical transmission apparatus, optical transmission system, and method of controlling excitation light frequency

ABSTRACT

An optical transmission apparatus includes a wavelength converter that wavelength-converts input signal light using a nonlinear optical medium to output the converted signal light, a memory that holds first information relating to a wavelength conversion characteristic of the wavelength converter, a communication interface that receives second information relating to a second wavelength conversion characteristic of an adjacent optical transmission apparatus, and a control circuit that determines, using the first information and the second information when the second information is received, an excitation light frequency at which a gain deviation of main signal light subjected to a wavelength conversion is minimized to set the determined excitation light frequency in the wavelength converter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2017-220987, filed on Nov. 16,2017, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical transmissionapparatus having a wavelength conversion function, an opticaltransmission system, and a method of controlling an excitation lightfrequency.

BACKGROUND

In order to cope with an increase in demand for communication lines, thewavelength multiplexing number is increased in the wavelength divisionmultiplexing (WDM), and the transmission rate per wavelength isincreased in the digital coherent technology. In the wavelength band,the 1.55 μm band (C band) matching the amplification wavelength band ofan erbium-doped fiber amplifier (EDFA) is most widely used. A techniqueusing a signal in the 1.59 μm band (L band) by shifting theamplification band of the EDFA to the long wavelength side has also beenput to practical use. As a method of further expanding the wavelengthband, research and development of optical amplifiers and so forth foruse in the 1.46 to 1.53 μm band (S band) is also conducted although suchoptical amplifiers have not yet been put into practical use.

On the other hand, technology for converting the signal light wavelengthby using the four wave mixing (FWM) effect in the optical fiber has beenstudied and developed for about 20 years. The FWM is a phenomenon inwhich a new wavelength which does not coincide with any of the incidentwavelengths occurs when light of two or more different wavelengths isincident on a highly nonlinear fiber (HNLF). Wavelength conversiontechniques using a phase conjugate and the FWM of a nonlinear opticalcrystal have also been offered. For nonlinear optical media such asnonlinear optical crystals and the HNLF, there is a wavelength at whichchromatic dispersion is zero. When the excitation light wavelengthcoincides with the zero dispersion wavelength, a broadband wavelengthconversion is possible.

In a wavelength conversion using the FWM or the phase conjugation, it ispossible to collectively convert WDM signals, but there are factors thatlimit the wavelength band. When the zero dispersion wavelength and theexcitation light wavelength do not coincide with each other, theconversion efficiency of the main signal decreases at a wavelength awayfrom that of the excitation light, and a Gain deviation or a tiltoccurs. In the following description, both the term “wavelength” and theterm “frequency” are used. Since the wavelength and the frequency are inreciprocal relation, the two has practically the same meaning.

FIGS. 1A to 1C illustrate the tilt generated in the main signal lightwhen the zero dispersion frequency and the excitation light frequency donot coincide with each other. FIG. 1A illustrates a simulation ofconversion efficiency when the zero dispersion frequency of an HNLF isdeviated to the minus side from the excitation light frequency. FIG. 1Billustrates a simulation of the conversion efficiency when the zerodispersion frequency of HNLF is deviated to the plus side from theexcitation light frequency. When the frequency deviation is 0 GHz (solidline), a flat conversion characteristic may be obtained over a wideband. As the frequency deviation increases, the signal light whosefrequency is away from the excitation light frequency has a lowconversion efficiency, and a gain deviation or a tilt occurs. FIG. 1C isa diagram illustrating the amount of degradation of the conversionefficiency of the outermost channel as a function of the frequencydeviation. As the frequency deviation increases, signal efficiencydeteriorates remarkably on both plus and minus sides.

FIG. 2 is a diagram for explaining the influence of a tilt which occursin a wavelength conversion. When performing wavelength conversionstwice, for example, when converting C band to L band on the transmissionside, and then converting the L band to the C band on the receptionside, the tilt accumulates, whereby the signal wavelength away from thewavelength of the excitation light is disadvantageous. When thewavelength conversion at a fixed excitation light wavelength isperformed by a wavelength converter using a nonlinear optical medium, atilt occurs in the conversion light as illustrated in FIG. 2. When theexcitation light wavelength is controlled to match with the zerodispersion wavelength, the occurrence of tilt is suppressed. However,due to production variations of the HNLF, the zero dispersion wavelengthof the wavelength converter on the transmission side may be differentfrom the zero dispersion wavelength of the wavelength converter on thereception side. In this case, the wavelength of the main signal enteringthe receiver deviates from the ITU grid, and there arises a problem thatthe main signal may not be received.

The following is a reference document.

[Document 1] Japanese Laid-open Patent Publication No. 2000-75330.

SUMMARY

According to an aspect of the embodiments, an optical transmissionapparatus includes a wavelength converter that wavelength-converts inputsignal light using a nonlinear optical medium to output the convertedsignal light, a memory that holds first information relating to awavelength conversion characteristic of the wavelength converter, acommunication interface that receives second information relating to asecond wavelength conversion characteristic of an adjacent opticaltransmission apparatus, and a control circuit that determines, using thefirst information and the second information when the second informationis received, an excitation light frequency at which a gain deviation ofmain signal light subjected to a wavelength conversion is minimized toset the determined excitation light frequency in the wavelengthconverter.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are diagrams illustrating a tilt generated in main signallight when a zero dispersion frequency does not coincide with anexcitation light frequency;

FIG. 2 is a diagram for explaining the influence of a tilt which occursin a wavelength conversion;

FIG. 3 is a basic configuration diagram of an optical transmissionsystem according to an embodiment;

FIG. 4 is a diagram illustrating an example of a wavelength converterused in an embodiment;

FIG. 5 is a diagram for explaining collective conversion of WDM signalsby a wavelength converter;

FIG. 6 is a basic flow of operations of an optical transmissionapparatus according to the embodiment;

FIG. 7 is a schematic diagram of an optical transmission systemaccording to a first embodiment;

FIG. 8 is a diagram for explaining the effect of setting an excitationlight frequency;

FIG. 9 is a flowchart of the operation of the optical transmissionapparatus of the first embodiment;

FIG. 10 is a schematic diagram of an optical transmission systemaccording to a second embodiment;

FIG. 11 is a diagram illustrating measurement of wavelength conversioncharacteristic information;

FIGS. 12A to 12C are diagram for explaining a change in the amount oftilt when the excitation light frequency is swept;

FIG. 13 is a diagram illustrating measurement results;

FIG. 14 is a diagram for explaining the effect of setting the excitationlight frequency;

FIG. 15 is a schematic diagram of an optical transmission systemaccording to a third embodiment;

FIG. 16 is a diagram illustrating a configuration example of an HNLFcharacteristic table;

FIG. 17 is a diagram for explaining a method of calculating excitationlight frequency;

FIG. 18 is a diagram for explaining the effect of setting the excitationlight frequency;

FIG. 19 is a schematic diagram of an optical transmission systemaccording to a fourth embodiment;

FIG. 20 is a diagram for explaining setting of an excitation lightfrequency according to a fifth embodiment;

FIG. 21 is a diagram for explaining setting of an excitation lightfrequency with a frequency difference within an allowable range;

FIG. 22 is a diagram for explaining the setting of an optimum excitationlight frequency with a frequency difference within an allowable range;

FIGS. 23A and 23B are schematic diagrams of a modification of theoptical transmission system; and

FIGS. 24A and 24B are schematic diagrams of another modification of theoptical transmission system.

DESCRIPTION OF EMBODIMENTS

The chromatic dispersion for a nonlinear optical medium, which is notlimited to HNLF, includes dispersion occurring due to the material anddispersion occurring due to the structure. With an optical fiber, a zerodispersion wavelength is controlled by adjusting the dispersionoccurring due to the structure by designing the refractive index profileof its cross section. The optical fiber is manufactured so that therefractive index profile is as uniform as possible by controllingmanufacturing conditions. However, the dispersion of the zero dispersionwavelength may not be made zero.

In the optical transmission apparatus according to an embodiment, byusing wavelength conversion characteristic information of the ownstation and wavelength conversion characteristic information of thepartner station, control is performed so that the excitation lightwavelength is made close to the zero dispersion wavelength of thenonlinear optical medium to be used. For example, the wavelengthconversion characteristic (hereinafter referred to as HNLFcharacteristic as appropriate) of a nonlinear optical medium such as theHNLF is measured in advance, and stored in a memory or the like. A unitis provided between a first-time wavelength conversion unit (wavelengthconversion unit on the transmission side) and a second-time wavelengthconversion unit (wavelength conversion unit on the reception side) tonotify the partner station of the HNLF characteristic of the ownstation. Upon receiving the HNLF characteristic, the opticaltransmission apparatus calculates an optimum excitation light frequencybased on the HNFL characteristic of the own station and the receivedHNLF characteristic, and sets the calculated frequency or wavelength tothe pump light source.

The optimum excitation light frequency is a frequency at which a gaindeviation of the main signal light having undergone the wavelengthconversions twice is minimized. For example, the optimum excitationlight frequency is a frequency at which the power ratio of the outputlight of the second-time wavelength converter with respect to the inputlight to the first-time wavelength converter, or deterioration of theconversion efficiency of the output light of the second-time wavelengthconverter with respect to the input light to the first-time wavelengthconverter is minimized.

Basic Configuration

FIG. 3 is a basic configuration diagram of an optical transmissionsystem 1 according to the embodiment. The optical transmission system 1includes an optical transmission apparatus 10A (A station), and anoptical transmission apparatus 10B (B station). The optical transmissionapparatus 10A and the optical transmission apparatus 10B have awavelength conversion function using a nonlinear optical effect. As anexample, the HNLF is used to perform the wavelength conversion function.The optical transmission apparatuses 10A and 10B are connected to eachother by optical transmission lines 2 and 3, and bidirectionalcommunication with each other is possible. In the following description,in order to distinguish the two optical transmission apparatuses 10, “A”is assigned to one optical transmission apparatus 10 and its constituentelements as the reference sign, and “B” is assigned to the other opticaltransmission apparatus 10 and its constituent elements as the referencesign. The two optical transmission apparatuses 10 have the identicalconfiguration, and are generically referred to without the alphabet asappropriate.

The optical transmission apparatus 10A includes a transponder 11A, awavelength conversion unit 13A, and HNLF characteristic communicationinterfaces (I/F) 14A and 15A. The wavelength conversion unit 13Aincludes a wavelength converter 131A and a wavelength converter 132A.The wavelength converters 131A and 132A are wavelength converters usingthe HNLF, and perform a wavelength conversion using pump light sources133A and 134A respectively. The pump light sources 133A and 134A arelight sources combining an EDFA with a DFB (Distributed Feedback) laserthat outputs narrowband excitation light of a single mode, for example.

The HNLF of the wavelength converter 131A of the optical transmissionapparatus 10A is denoted as HNLF1, and the excitation light frequency ofthe pump light source 133A is denoted as νp1. The HNLF of the wavelengthconverter 132A is denoted as HNLF4, and the excitation light frequencyof the pump light source 134A is denoted as νp4.

The wavelength conversion unit 13A of the optical transmission apparatus10A includes an excitation light frequency control unit 135A and an HNLFcharacteristic holding memory 136A. The HNLF characteristic holdingmemory 136A holds characteristic information of the HNLF1 andcharacteristic information of the HNLF4. The excitation light frequencycontrol unit 135A calculates the excitation light frequencies νp1 andνp4 to be set to the pump light sources 133A and 134A respectively basedon the information stored in the HNLF characteristic holding memory136A, and the HNLF characteristic information of the opposed opticaltransmission apparatus 10B.

The basic configuration of the optical transmission apparatus 10B is thesame as that of the optical transmission apparatus 10A, and includes atransponder 11B, a wavelength conversion unit 13B, and HNLFcharacteristic communication interfaces 14B and 15B. The HNLF of awavelength converter 132B of the optical transmission apparatus 10B isdenoted as HNLF2, and the excitation light frequency of the pump lightsource 134 is denoted as νp2. The HNLF of the wavelength converter 131on the transmission side of the optical transmission apparatus 10B tothe network is denoted as HNLF3, and the excitation light frequency ofthe pump light source 133 is denoted as νp3.

An HNLF characteristic holding memory 136B of the wavelength conversionunit 13B of the optical transmission apparatus 10B holds characteristicinformation of the HNLF3, and characteristic information of the HNLF2.An excitation light frequency control unit 135B of the opticaltransmission apparatus 10B calculates excitation light frequencies νp3and νp2 to be set to the pump light sources 133B and 134B respectivelybased on the information stored in the HNLF characteristic holdingmemory 136B and the HNLF characteristic information of the opposedoptical transmission apparatus 10A.

Between the optical transmission apparatuses 10A and 10B, the excitationlight frequencies νp1 and νp2 are calculated to values at which the gaindeviation or the tilt of the main signal light having undergone thewavelength conversion (first-time wavelength conversion) by thewavelength converter 131A and the wavelength conversion (second-timewavelength conversion) by the wavelength converter 132B is minimized.

Similarly, the excitation light frequencies νp3 and νp4 are calculatedto values at which the gain deviation or the tilt of the main signallight having undergone the wavelength conversion (first-time wavelengthconversion) by a wavelength converter 131B and the wavelength conversion(second-time wavelength conversion) by the wavelength converter 132A isminimized.

The optical transmission apparatus 10, through the HNLF characteristiccommunication interfaces 14 and 15, notifies the partner station of theHNLF characteristic information of the own station to share the HNLFcharacteristic information of the own station with the partner station.The NHLF characteristic holding memory 136 and the excitation lightfrequency control unit 135 are not necessarily arranged inside thewavelength conversion unit 13, and may be implemented by a memory and aprocessor which are arranged inside of each device. For simplicity ofillustration, a single transponder 11 is depicted in the opticaltransmission apparatus 10. However, a plurality of transponders 11 isgenerally arranged in each optical transmission apparatus 10, and aplurality of client signals is transmitted and received by the WDMmethod. A multiplexer (MUX) is inserted between the plurality oftransponders 11 and the wavelength converter 131, and a demultiplexer(DEMUX) is inserted between the plurality of transponders 11 and thewavelength converter 132.

The operation of the optical transmission system 1 after the excitationlight frequency is set in the optical transmission apparatus 10A and theoptical transmission apparatus 10B is as follows. The wavelengthconverter 131A of the optical transmission apparatus 10A converts signallight ν1 input from the transponder 11A into signal light ν2 ofdifferent wavelength using excitation light νp1. The signal light ν2 istransmitted to the optical transmission apparatus 10B through theoptical transmission line 2, and input to the wavelength converter 132Bof the optical transmission apparatus 10B. The wavelength converter 132Bconverts the received signal light ν2 into the signal light ν3 of theoriginal wavelength using the excitation light νp2, and outputs it tothe transponder 11B.

In reverse communication, the wavelength converter 131B of the opticaltransmission apparatus 10B convert the signal light ν4 input from thetransponder 11B into the signal light ν5 of the different wavelengthusing the excitation light νp3. The signal light ν5 is transmitted tothe optical transmission apparatus 10A through the optical transmissionline 3, and input to the wavelength converter 132A of the opticaltransmission apparatus 10A. The wavelength converter 132A converts thereceived signal light ν5 into the signal light ν6 of the originalwavelength using the excitation light νp4, and outputs it to thetransponder 11A.

Each of the optical transmission apparatus 10A and the opticaltransmission apparatus 10B autonomously sets the optimum excitationlight wavelength for the pump light sources 133 and 134 of the ownstation using the wavelength conversion characteristic information ofthe own station, and the wavelength conversion characteristicinformation of the partner station. As a result, the gain deviation ortilt of the main signal light having undergone the wavelength conversiontwice is minimized, and wideband wavelength conversion is implemented.The frequency bands of the optical transmission lines 2 and 3 areeffectively used, and the high quality optical transmission system 1 isprovided.

Circumstances of Calculation or Recalculation of Excitation LightFrequency

The relationship of the wavelength conversion efficiency between theopposed optical transmission apparatuses 10 changes when any one of theoptical transmission apparatuses 10 has been newly introduced into theoptical transmission system 1 (when the connection topology haschanged), or when the wavelength converter 131 or 132 is exchanged inany one of the optical transmission apparatuses 10. When an event occursin which the characteristic (zero dispersion frequency and so forth) ofthe wavelength converter may change, for example, when the wavelengthconverter 131 (or 132) is replaced, or when a new device is introduced,the excitation light frequencies of the pump light sources 133 and 134are recalculated.

The excitation light frequency may be recalculated at the time ofactivation of any one of the optical transmission apparatuses 10 afterits repair/maintenance. In a case where wavelength conversioncharacteristic information is transmitted and received using opticalsupervisory channel (OSC), the excitation light frequency may berecalculated at the time of recovery from optical transmission linedisconnection, recovery from an interruption of the OSC signal,replacement of the failed OSC module, or the like. In a case of opticaltransmission via a relay station, the excitation light frequency isrecalculated at the time of recovery from the optical transmission linedisconnection in the relay section, or at the time of recovery from aninterruption of the OSC signal.

In a case where the HNLF characteristic information is transferred bythe network management operation system (NE-OPS), an opticaltransmission apparatus 10 or the wavelength conversion unit 13 thatperforms the wavelength conversion is not monitored by the NE-OPS, andeven when restoration is carried out thereafter, the excitation lightfrequency is recalculated.

When an event which may change the wavelength conversion characteristicof the wavelength converter occurs, the optical transmission apparatus10 outputs the HNLF characteristic information stored in the HNLFcharacteristic holding memory 136 to a supervisory channel or asupervisory path repeatedly. At the same time, the optical transmissionapparatus 10 waits for the reception of the HNLF characteristicinformation from the partner station.

When HNLF characteristic information is exchanged between two opposedoptical transmission apparatuses 10, the optical transmission apparatus10, using the HNLF characteristic information of the own station and thereceived HNLF characteristic information, determines the optimumexcitation light frequency for the wavelength converters 131 and 132which is used in the own station. The timing of calculation orrecalculation of the excitation light frequency will be described againin each embodiment described later.

FIG. 4 illustrates a configuration example of the wavelength converter131 (or 132) used in the optical transmission apparatuses 10A and 10B.The signal light Es of the wavelength λs input to a port 102 of thewavelength converter 131 is multiplexed with the excitation light Epfrom the pump light source 133 by a WDM coupler 106, and input to a port108 a of an optical circulator 108. The multiplexed signal is outputfrom a port 108 b of the optical circulator 108, and input to a port 110a of a polarization beam splitter (PBS) 110. In the PBS110, the signallight is divided into the TE polarized wave and the TM polarized wave,which are output to ports 110 c and 110 b respectively.

The ports 110 c and 110 b of the PBS110 are connected to an HNLF112. TheHNLF112 has two main axes. The port 110 C of the PBS110 is connected tothe HNLF112 at an angle at which the polarized wave in TE mode matcheswith the first principal axis at one end of the HNLF112. The port 110 bof the PBS110 is connected to the HNLF112 at an angle such that thepolarized wave in TM mode matches with the first principal axis at theother end of the HNLF112. The polarized wave in the TE mode and thepolarized wave in the TM mode are subjected to the wavelength conversionby FWM in the HNLF112, and the wavelength conversion light Ec isgenerated. The PBS110 also serves as a polarization beam combiner. Thewavelength conversion light Ec is input to the port 108 b of the opticalcirculator 108 from the port 110 a of the PBS110. This signal is outputto a port 108 c of the optical circulator 108. The excitation light Epand the input light Es are removed by an optical wavelength filter 114,and optical signal Ec having a wave length λc is output.

Instead of the configuration of FIG. 4, after dividing the input signallight (for example, C band light) into TE polarized wave (orthogonalpolarized wave) and TM polarized wave (horizontal polarized wave),excitation light branched into respective polarization components may bemultiplexed, and each multiplexed wave may be wavelength-converted by anonlinear optical medium. In this case, after each residual excitationlight is removed from the wavelength conversion light of each polarizedwave, and the wavelength conversion light is polarization multiplexed,the wavelength conversion light (for example, L band light) may beoutput.

FIG. 5 is a diagram illustrating collective conversion of WDM signals bythe wavelength converter 131 (or 132). Optical signals of the differentwavelength input from the plurality of transponders 11 may becollectively wavelength-converted by the wavelength converter 131. WDMsignal light is obtained by wavelength division multiplexing N channeloptical signals having different wavelengths λ1, λ2 . . . λN, where λ1is the shortest wavelength, and λN is the longest wavelength. Theexcitation light wave length λp is set lower than the wave length λ1.The WDM signal light is converted into conversion light by the FWM usingthe excitation light. The signal light includes N-channel conversionlight signals having different wavelengths λ1′, λ2′ . . . λN′. In eachchannel, the arrangement of the input signal specific light and theconversion light is symmetrical with respect to the wave length λp ofthe pump light.

Information on the wavelength conversion characteristic (HNLFcharacteristic) is mutually exchanged between the optical transmissionapparatus 10A and the optical transmission apparatus 10B, and an optimumexcitation light wavelength at which the chromatic dispersion or thetilt is minimized from information of the own station and information ofthe partner station is determined. Therefore, a flat conversionefficiency may be obtained with the optical signal of each channel, andthe influence of tilt may be reduced.

Although FIG. 5 illustrates the conversion from the long wavelength bandto the short wavelength band, equivalent conversion efficiency may beobtained in the same way even by conversion from the short wavelengthband to the long wavelength band.

FIG. 6 is a basic flow of the operation of the optical transmissionapparatus 10B. It is assumed that the optical transmission apparatus 10Bis newly introduced. When the power is turned on, the opticaltransmission apparatus 10B waits for the reception of HNLFcharacteristic information from the partner station (S11). At the sametime, the optical transmission apparatus 10B repeatedly outputs the HNLFcharacteristic information owned by the own station. The HNLFcharacteristic information is an example of wavelength conversioncharacteristic information. When the optical transmission apparatus 10Bis introduced into the system, the optical transmission apparatus 10Bdoes not recognize which device is the partner station, and theexcitation wavelengths of the pump light sources 133B and 134B remainunadjusted.

Upon receiving the HNLF characteristic information from the partnerstation (“YES” in S11), the excitation light frequency at which the gaindeviation or the tilt of the main signal light having undergone thewavelength conversion twice is minimized is calculated from the receivedHNLF characteristic information and the HNLF characteristic informationof the own station (S12). The calculated excitation light frequency isset to the pump light sources 133 and 134, and the excitation light isoutput (S13). Thereafter, the input signal light is subjected to thewavelength conversion and output (S14). When receiving thewavelength-converted signal light ν2 from the network side, the signallight ν2 is wavelength-converted using the excitation light, and thesignal light ν3 of the original wavelength is output to the transponder11. When the signal light ν4 transmitted from the transponder 11 to thenetwork is input, the signal light ν4 is wavelength-converted, and thewavelength-converted signal light ν5 is output to the opticaltransmission line 3.

The same processing is performed in the optical transmission apparatus10A. Referring to FIG. 3, both the optical transmission apparatus 10Aand the optical transmission apparatus 10B wait for the reception ofHNLF characteristic information from the partner station whilerepeatedly outputting the HNLF characteristic information of the ownstation. When the HNLF characteristic information from the partnerstation is obtained, the excitation light frequency of the pump lightsources 133 and 134 of the own station is set to the optimum frequencyusing the acquired HNLF characteristic information and the HNLFcharacteristic information of the own station.

First Embodiment

FIG. 7 is a schematic diagram of an optical transmission system 1-1according to a first embodiment. The optical transmission system 1-1includes an optical transmission apparatus 10A-1 (A station) and anoptical transmission apparatus 10B-1 (B station). The basicconfiguration of the optical transmission system 1-1 is the same as thebasic configuration of FIG. 3. The features according to the firstembodiment will be mainly described.

In the first embodiment, the zero dispersion frequency of the wavelengthconverters 131 and 132 is stored in the HNLF characteristic holdingmemory 136 as wavelength conversion characteristic information. The HNLFcharacteristic holding memory 136A of an optical transmission apparatus10A-1 stores the zero dispersion frequency νz1 of the HNLF1 of thewavelength converter 131A, and the zero dispersion frequency νz4 of theHNFL4 of the wavelength converter 132A. The HNLF characteristic holdingmemory 136B of an optical transmission apparatus 10B-1 stores the zerodispersion frequency νz3 of the HNLF3 of the wavelength converter 131B,and stores the zero dispersion frequency νz2 of the HNFL2 of thewavelength converter 132B.

A supervisory control light transmitting unit 141 and a supervisorycontrol light receiving unit 142 are used as the HNLF characteristiccommunication interfaces 14 and 15. The HNLF characteristic informationis transmitted and received using the optical supervisory channel (OSC).One wavelength of WDM is assigned to the OSC, and the OSCbidirectionally connects adjacent nodes. The supervisory control lighttransmitting unit 141 and the supervisory control light receiving unit142 may be implemented by one OSC card or one OSC module.

An optical amplifier 16 and a WDM coupler 19 are arranged at the outputstage of the wavelength conversion unit 13 to the network. The zerodispersion frequency information output from the supervisory controllight transmitting unit 141 is output to the same output port as thesignal light through the WDM coupler 19, and is sent out to the opticaltransmission line 2. An optical amplifier 17 and a WDM coupler 20 arearranged at the input stage of the wavelength conversion unit 13 fromthe network. The zero dispersion frequency information received from theoptical transmission line 3 is demultiplexed by the WDM coupler 20, andinput to the supervisory control light receiving unit 142.

Upon receiving the zero dispersion frequencies νz1 and νz4 from theoptical transmission apparatus 10A-1, the optical transmission apparatus10B-1 reads out zero dispersion frequencies νz2 and νz3 of the ownstation from the HNLF characteristic holding memory 136B. The excitationlight frequency control unit 135B calculates and sets the excitationlight frequency νp3 of the pump light source 133B and the excitationlight frequency νp2 of the pump light source 134B using the zerodispersion frequencies νz1, νz4, νz2, and νz3.

The excitation light frequencies νp2 and νp3 of the optical transmissionapparatus 10B-1 are calculated by the equation (1).νp2=(νz1+νz2)/2νp3=(νz3+νz4)/2   (1)

The excitation frequency νp2 of the pump light source 134B is set to anaverage value of the zero dispersion frequency νz1 of the wavelengthconverter 131A of the opposed optical transmission apparatus 10A-1, andthe zero dispersion frequency νz2 of the wavelength converter 132B ofthe own station.

The excitation frequency νp3 of the pump light source 133B is set to anaverage value of the zero dispersion frequency νz3 of the wavelengthconverter 131B of the own station, and the zero dispersion frequency νz4of the wavelength converter 132A of the opposed optical transmissionapparatus 10A-1.

Upon receiving the zero dispersion frequencies νz2 and νz3 from theoptical transmission apparatus 10B-1, the optical transmission apparatus10A-1 calculates and sets the excitation light frequency νp1 of the pumplight source 133A, and the excitation light frequency νp4 of the pumplight source 134A using the received zero dispersion frequencies νz2 andνz3, and the zero dispersion frequencies νz1 and νz4 of the own station.

The excitation light frequencies νp1 and νp4 of the optical transmissionapparatus 10A-1 are calculated by the equation (2).νp1=(νz1+νz2)/2νp4=(νz3+νz4)/2   (2)

The excitation frequency νp1 of the pump light source 133A is set to anaverage value of the zero dispersion frequency νz1 of the wavelengthconverter 131A of the own station, and the zero dispersion frequency νz2of the wavelength converter 132B of the opposed optical transmissionapparatus 10B-1.

The excitation frequency νp4 of the pump light source 134A is set to anaverage value of the zero dispersion frequency νz3 of the wavelengthconverter 131B of the opposed optical transmission apparatus 10B-1, andthe zero dispersion frequency νz4 of the wavelength converter 132A ofthe own station.

As may be seen from equations (1) and (2), νp1=νp2, and νp4=νp3. Thesame excitation light frequency at which the deterioration of the mainsignal light after the conversion has been performed twice between theoptical transmission apparatus 10A-1 and the optical transmissionapparatus 10B-1 is minimized is set, whereby it is possible to receivemain signal light with less deterioration in the ITU grid.

FIG. 8 is a diagram for explaining the effect of setting the excitationlight frequency. The horizontal axis represents the excitation lightfrequency, and the vertical axis represents the amount of tilt. Theamount of tilt of the HNLF1 of the optical transmission apparatus 10A-1is minimized at the zero dispersion frequency νz1, and increases as itgoes away from the zero dispersion frequency νz1. Although the tiltcharacteristic of the HNLF2 of the optical transmission apparatus 10B-1is not the same as that of the HNFL1, the tendency of the tiltcharacteristic of the HNLF2 is the same as that of the HNFL1. The amountof tilt is the minimum at the zero dispersion frequency νz2, andincreases as it goes away from the zero dispersion frequency νz2.

The solid line in FIG. 8 illustrates the characteristic of the totaledtilt of the HNLF1 and the HNLF2. The characteristic of the totaled tiltis minimized at the midpoint or the average point of the zero dispersionfrequencies νz1 and νz2. As illustrated in FIG. 8, although thecharacteristic curves of the HNLF1 and the HNLF2 are almost symmetrical,and are similar in shape, they have different zero dispersionfrequencies. In this case, by setting the excitation light frequency νp2to the average value of the zero dispersion frequency, it is possible tosuppress the tilt and keep the wavelength conversion efficiency flat.

In the optical transmission system 1-1, the excitation light frequencyis recalculated at the time of replacement of the failed supervisorycontrol light transmitting unit 141 or the failed supervisory controllight receiving unit 142 in addition to when the wavelength converter131 or 132 is replaced, and when a new optical transmission apparatus 10is introduced. The excitation light frequency is recalculated at thetime of recovery from the interruption of the OSC signal, and at thetime of recovery from disconnection of the optical transmission line 2or 3. The fundamental flow of the wavelength conversion andtransmission/reception of the optical signal after setting theexcitation light frequency is as follows.

In the optical transmission apparatus 10A-1, the WDM signals ν1 obtainedby wavelength division multiplexing the client signals from theplurality of transponders 11A with the optical multiplexer/demultiplexer(MUX/DEMUX) 12A are collectively converted into the signal light ν2 inthe wavelength converter 131A. The signal light ν2 is amplified by anoptical amplifier 16A, passes through a WDM coupler 19A, and is outputto the optical transmission line 2.

In the optical transmission apparatus 10B-1, the signal light ν2received after passing through a WDM coupler 20B is amplified by anoptical amplifier 17B, and input to the wavelength conversion unit 13B.The signal light ν2 is collectively converted into the signal light ν3by the wavelength converter 132B. The components of respectivewavelengths included in the signal light ν3 are substantially equal tothe components of the respective wavelengths included in the transmittedWDM signal ν1. The signal light ν3 is separated into signals withrespective wavelengths by an optical multiplexer/demultiplexer 12B, andis supplied to the corresponding transponder 11B.

In reverse communication, the wavelength converter 131B of the opticaltransmission apparatus 10B-1 convert the signal light ν4 input from thetransponder 11B into the signal light ν5 of the different wavelength.The signal light ν5 is amplified by the optical amplifier 16B, passesthrough a WDM coupler 19B, and is output to the optical transmissionline 3. The signal light ν5 received by the optical transmissionapparatus 10A-1 passes through a WDM coupler 20A, is amplified by anoptical amplifier 17A, and is input to the wavelength converter 132A.The wavelength converter 132A converts the input signal light ν5 intothe signal light ν6 of the original wavelength, and outputs it to thetransponder 11A.

In any directional communication, since the excitation light frequenciesof the pump light sources 133 and 134 is set to a frequency at which thetotal amount of tilt is minimized, a flat wavelength conversionefficiency may be obtained by suppressing the tilt of the main signallight having undergone the wavelength conversion twice.

FIG. 9 illustrates an operation flow of the optical transmissionapparatus 10B-1 according to the first embodiment. When an event thatmay change a wavelength conversion characteristic in relation to thepartner station, such as replacement of wavelength converter 131 or 132,or recovery from optical transmission line disconnection, occurs, theoptical transmission apparatus 10B-1 waits for the reception of the zerodispersion frequency (S21). Simultaneously, the zero dispersionfrequency of the own station may be repeatedly output. Upon receivingthe zero dispersion frequency from the partner station (“YES” in S21),excitation light frequencies (νp2, νp3) are calculated from equations(1) and (2) using the received zero dispersion frequencies (νz1, νz4),and the zero dispersion frequencies (νz2, νz3) of the own station (S22).

After setting the calculated excitation light frequency to the pumplight source, the excitation light is output (S23), and the input signallight is subjected to the wavelength conversion using the excitationlight and output (S24).

The same operation flow is performed in the optical transmissionapparatus 10A-1. According to this method, an optimum excitation lightfrequency is set in both the optical transmission apparatus 10A-1 andthe optical transmission apparatus 10B-1 which are opposed to eachother. It is possible to minimize the tilt of the main signal havingundergone the wavelength conversion twice.

Second Embodiment

FIG. 10 is a schematic diagram of an optical transmission system 1-2according to an second embodiment. The optical transmission system 1-2includes an optical transmission apparatus 10A-2 (A station) and anoptical transmission apparatus 10B-2 (B station). The basicconfiguration of the optical transmission system 1-2 is the same as thebasic configuration of the optical transmission system 1-1 according tothe first embodiment. The features of the second embodiment will mainlybe described.

In the second embodiment, information indicating the wavelengthconversion characteristic of the wavelength converters 131 and 132 by anapproximate expression or a function is held as the wavelengthconversion characteristic information. As in the first embodiment, theHNLF characteristic is used as an example of the wavelength conversioncharacteristic. The coefficients of the approximate curve of thequadratic function are stored in the HNLF characteristic holding memory136 as information of the approximate expression or the function.

As in the first embodiment, the supervisory control light transmittingunit 141 and the supervisory control light receiving unit 142 are usedas the HNLF characteristic communication interfaces 14 and 15. Thecoefficient information of the approximate curve representing the HNLFcharacteristic is transmitted and received between the opticaltransmission apparatus 10A-2 and the optical transmission apparatus10B-2 using the OSC.

When the generated the amount of tilt y is expressed as a quadraticfunction of the excitation light frequency x, the amount of tilt isexpressed by the following expression.y=ax ² +b x+c  (3)For each of the wavelength converters 131 and 132, a set of coefficients(a, b, c) is held as HNLF characteristic information.

The HNLF characteristic holding memory 136A of the optical transmissionapparatus 10A-2 holds the coefficients (a1, b1, c1) as the wavelengthconversion characteristic of the HNLF1, and holds the coefficients (a4,b4, c4) as the wavelength conversion characteristic of the HNLF4.

The HNLF characteristic holding memory 136B of the optical transmissionapparatus 10B-2 holds the coefficients (a2, b2, c2) as the wavelengthconversion characteristic of the HNLF2, and holds the coefficients (a3,b3, c3) as the wavelength conversion characteristic of the HNLF3.

The coefficient information output from a supervisory control lighttransmitting unit 141A of the optical transmission apparatus 10A-2 isoutput to the same output port as that of the signal light through theWDM coupler 19A, and is sent out to the optical transmission line 2. Thecoefficient information (a1, b1, c1) and (a4, b4, c4) from the opticaltransmission apparatus 10A-2 is received by a supervisory control lightreceiving unit 142B of the optical transmission apparatus 10B-2 throughthe WDM coupler 20B, and is supplied to the excitation light frequencycontrol unit 135B. The excitation light frequency control unit 135Breads the coefficient information (a2, b2, c2) and (a3, b3, c3) of theown station from the HNLF characteristic holding memory 136B. Theexcitation light frequency control unit 135B calculates and sets theexcitation light frequency νp3 of a pump light source 133B, and theexcitation light frequency νp2 of the pump light source 134B using thecoefficient information (a1, b1, c1), (a4, b4, c4), (a2, b2, c2), and(a3, b3, c3).

Similarly, the coefficient information (a2, b2, c2) and (a3, b3, c3)output from a supervisory control light transmitting unit 141B of theoptical transmission apparatus 10B-2 is transmitted from the WDM coupler19B to a transmission line 3. The coefficient information (a2, b2, c2)and (a3, b3, c3) of the optical transmission apparatus 10B-2 is receivedby a supervisory control light receiving unit 142A of the opticaltransmission apparatus 10A-2 through the WDM coupler 20A, and issupplied to the excitation light frequency control unit 135A. Theexcitation light frequency control unit 135A reads the coefficientinformation (a1, b1, c1) and (a4, b4, c4) of the own station from theHNLF characteristic holding memory 136A. The excitation light frequencycontrol unit 135A calculates and sets the excitation light frequency νp1of the pump light source 133A, and the excitation light frequency νp4 ofthe pump light source 134A using the coefficient information (a1, b1,c1), (a4, b4, c4), (a2, b2, c2), and (a3, b3, c3).

The coefficient information of the respective wavelength converters 131and 132 of each optical transmission apparatus 10-2 is measured inadvance at the time of manufacture, and is stored in the HNLFcharacteristic holding memory 136.

FIG. 11 is a diagram for explaining measurement of wavelength conversioncharacteristic information. A test personal computer (PC) is connectedto the wavelength conversion unit 13. The excitation light frequency ofthe pump light source 133 of the wavelength converter 131 is sweptthrough the excitation light frequency control unit 135. Signal lightνi1 and νi2 is input to the wavelength converter 131, and is subjectedto the wavelength conversion after combined with the excitation light.The outputs νo1 and νo2 of the wavelength converter 131 are monitoredwith an optical spectrum analyzer. The monitor result is recorded in thetest PC in association with the excitation light frequency. The samemeasurement is also performed for the wavelength converter 132. Themeasurement result is recorded in the test PC.

FIG. 12 is a diagram for explaining a change in the amount of tilt whenthe excitation light frequency is swept. The horizontal axis of eachchart represents frequency and the vertical axis represents power. Theupper part of FIG. 12 illustrates the output characteristic when theexcitation light frequency is larger than the zero dispersion frequency.The broken line arrow at the center represents the excitation light.There is a difference in the powers of the output conversion light νo1and νo2 with respect to the input signal light νi1 and νi2. The power ofthe conversion light νo2 having a frequency far from the excitationlight frequency decreases. The difference between the powers of theconversion light νo1 and νo2 is taken as the amount of tilt.

The middle part of FIG. 12 illustrates the output characteristic whenthe excitation light frequency coincides with the zero dispersionfrequency. The solid arrow at the center represents the zero dispersionfrequency and the excitation light frequency. Conversion light νo1 andνo2 having the same power is output with respect to the input signallight νi1 and νi2. The TILT does not occur.

The lower part of FIG. 12 illustrates the output characteristic when theexcitation light frequency is smaller than the zero dispersionfrequency. The broken arrow at the center represents the excitationlight. A difference in the powers of the output conversion light νo1 andνo2 with respect to the input signal light νi1 and νi2 respectivelyoccurs. The power of the conversion light μo2 having a frequency farfrom the excitation light frequency greatly decreases. The amount oftilt observed is recorded as a function of the excitation lightfrequency.

FIG. 13 is a diagram illustrating measurement results. The wavelengthconversion characteristic curve of the wavelength converter 131 isobtained as a function of the excitation light frequency. This curve isapproximated by the quadratic function y=ax²+bx+c by least squaresmethod. The coefficients (a, b, c) are stored in the HNLF characteristicholding memory 136.

In the optical transmission apparatus 10B-2, the excitation lightfrequency νp2 is calculated as the value of x at which the amount oftilt y12 in the equation (4) is minimized, and the excitation lightfrequency νp3 is calculated as the value of x at which the amount oftilt y34 in the expression (4) is minimized.y12=y1+y2=a1x ² +b1x+c1+a2x ² +b2x+c2y34=y3+y4=a3x ² +b3x+c3+a4x ² +b4x+c4   (4)

FIG. 14 is a diagram for explaining the effect of setting the excitationlight frequency. The horizontal axis represents the excitation lightfrequency, and the vertical axis represents the amount of tilt. Thecurve of the tilt characteristic of the HNLF1 of the opticaltransmission apparatus 10A-2, and the curve of the tilt characteristicof the HNLF2 of the optical transmission apparatus 10B-2 are asymmetricwith respect to the zero dispersion frequency at which the amount oftilt is minimized, and the degree of opening of the parabola(coefficient “a”) is different. In this case, by having the coefficients(a, b, c) of the approximate curve, it is possible to express eachwavelength conversion characteristic approximately.

The solid line in FIG. 14 represents the tilt characteristic y12 of thesum of the amount of tilt y1 of the HNLF1 and the amount of tilt y2 ofthe HNLF2. In the optical transmission apparatus 10B-2, the excitationlight frequency νp2 at which the total amount of tilt is minimizedbetween the optical transmission apparatus 10A-2 and the opticaltransmission apparatus 10B-2 is the optimum excitation light frequencyfor the wavelength converter 132B. In this case, the gain deviation ofthe main signal light having undergone the wavelength conversion twiceis minimized.

In the wavelength converter 131B of the optical transmission apparatus10B-2, the excitation light frequency νp3 at which the amount of tilty34 of the sum of the amount of tilt y3 of the HNLF3 and the amount oftilt y4 of the HNLF4 are minimized, is determined as the optimumexcitation light wave number.

Also in the optical transmission apparatus 10A-2, the excitation lightfrequency νp1 of the wavelength converter 131A and the excitation lightfrequency νp4 of the wavelength converter 132A are calculated accordingto the same procedure using the coefficient information received fromthe optical transmission apparatus 10B-2. As a result, νp1=νp2, νp4=νp3.The excitation light frequencies νp1 and νp4 are set to the pump lightsources 133A and 134A respectively.

In the second embodiment, even if the shapes or degrees of opening(coefficient “a”) of the characteristic curves of the HNLF1 and theHNLF2, or the HNLF4 and the HNLF3 are different, it is possible toselect the excitation light wavelength at which the amount of tilt isminimized as long as the difference between the zero dispersionfrequencies of the HNLF1 and the HNLF2 is small so as to be within therange where the quadratic function approximation holds.

In the optical transmission system 1-2 according to the secondembodiment, the excitation light frequency is recalculated at the timeof replacement of the failed supervisory control light transmitting unit141 or the failed supervisory control light receiving unit 142 inaddition to when the wavelength converter 131 or 132 is replaced, andwhen a new optical transmission apparatus 10 is introduced. Theexcitation light frequency is recalculated at the time of recovery fromthe interruption of the OSC signal, and at the time of recovery fromdisconnection of the optical transmission line 2 or 3. The fundamentalflow of the wavelength conversion and transmission/reception of theoptical signal after setting the excitation light frequency is the sameas that of the optical transmission system 1-1 according to the firstembodiment.

Also in the second embodiment, an excitation light frequency that is thesame excitation light frequency between the optical transmissionapparatus 10A-2 and the optical transmission apparatus 10B-2 and atwhich the tilt of the main signal light that has undergone thewavelength conversion twice between the optical transmission apparatus10A-2 and the optical transmission apparatus 10B-2 is minimized isautonomously set. As a result, the main signal light may be receivedwith uniform wavelength conversion efficiency within the ITU grid.

Third Embodiment

FIG. 15 is a schematic diagram of an optical transmission system 1-3according to a third embodiment. The optical transmission system 1-3includes an optical transmission apparatus 10A-3 (A station) and anoptical transmission apparatus 10B-3 (B station). The basicconfiguration of the optical transmission system 1-3 is the same as thebasic configuration of the optical transmission system 1-1 according tothe first embodiment and the basic configuration of the opticaltransmission system 1-2 according to the second embodiment. The featuresaccording to the third embodiment will be mainly described.

In the third embodiment, an HNLF characteristic table 138 describing thewavelength conversion characteristic of wavelength converters 131 and132 of the own station is stored in HNLF characteristic holding memory136. An HNLF characteristic table 138A of the optical transmissionapparatus 10A-3 includes an HNLF1 table describing the wavelengthconversion characteristic of the HNLF1, and an HNLF4 table describingthe wavelength conversion characteristic of the HNLF4. An HNLFcharacteristic table 138B of the optical transmission apparatus 10B-3includes an HNLF2 table describing the wavelength conversioncharacteristic of the HNLF2, and an HNLF3 table describing thewavelength conversion characteristic of the HNLF3.

As in the first embodiment and the second embodiment, the supervisorycontrol light transmitting unit 141 and the supervisory control lightreceiving unit 142 are used as the HNLF characteristic communicationinterfaces 14 and 15. The information of the HNLF characteristic table138 is transmitted and received between the optical transmissionapparatus 10A-2 and the optical transmission apparatus 10B-2 using theoptical supervisory channel (OSC).

FIG. 16 illustrates an example of an HNLF characteristic table 138A ofthe optical transmission apparatus 10A-3. The amount of tilt of theHNLF1 (dB) and the amount of tilt of the HNFL4 (dB) are described inassociation with the excitation light frequency. The HNLF1 table isformed by the excitation light frequency and the amount of tilt of theHNLF1, and the HNLF4 table is formed by the excitation light frequencyand the amount of tilt of the HNLF4.

The HNLF characteristic table 138B of the optical transmission apparatus10B-3 describes the amount of tilt of the HNLF2 and the amount of tiltof the HNFL3 in association with the excitation light frequency. TheHNLF tables 138A and 138B indicates the actual wavelength conversioncharacteristic more accurately than the approximate curve by fitting. Asin the second embodiment, the table information is obtained by sweepingthe excitation light frequency and measuring individual characteristicswhen manufacturing the wavelength converters 131 and 132.

The table information output from the supervisory control lighttransmitting unit 141A of the optical transmission apparatus 10A-3(HNLF1 table, HNFL4 table) is output to the same output port as that ofthe signal light through the WDM coupler 19, and is sent out to theoptical transmission line 2. The table information of the opticaltransmission apparatus 10A-3 is received by the supervisory controllight receiving unit 142B of an optical transmission apparatus 10B-3,and is supplied to the excitation light frequency control unit 135B. Theexcitation light frequency control unit 135B reads table information ofthe own station (HNLF2 table, HNLF3 table) from the HNLF characteristicholding memory 136B. The excitation light frequency control unit 135Bcalculates and sets the excitation light frequency νp3 of the pump lightsource 133B, and the excitation light frequency νp2 of the pump lightsource 134B using the table information.

The table information output from the supervisory control lighttransmitting unit 141B of the optical transmission apparatus 10B-2(HNLF2 table, HNLF3 table) is transmitted from the WDM coupler 19 to theoptical transmission line 3, and received by the supervisory controllight receiving unit 142A of the optical transmission apparatus 10A-2.The table information is supplied to the excitation light frequencycontrol unit 135A. The excitation light frequency control unit 135Areads the table information of the own station from the HNLFcharacteristic holding memory 136A, and calculates and sets theexcitation light frequency νp1 of the pump light source 133A, and theexcitation light frequency νp4 of the pump light source 134A.

FIG. 17 is a diagram for explaining a method of determining theexcitation light frequency performed in the optical transmissionapparatus 10B-3. Upon receiving the table information from the opticaltransmission apparatus 10A-3, the optical transmission apparatus 10B-3calculates the total amount of tilt for each excitation frequency usingthe received HNFL1 table and the HNFL2 table of the own station. Theexcitation light frequency (191.300 THz) at which the total amount oftilt is minimized is selected as the excitation light frequency νp2 ofthe pump light source 134B. This processing corresponds to thedetermination of the excitation light frequency x at which the amount oftilt y12 of the expression (4) is minimized in the second embodiment.

The optical transmission apparatus 10B-3 calculates the total amount oftilt for each excitation frequency using the received HNFL4 table andthe HNFL3 table of the own station, and selects as the excitation lightfrequency νp3 of the pump light source 133B the excitation lightfrequency at which the total amount of tilt is minimized. Thisprocessing corresponds to the determination of the excitation lightfrequency x at which the amount of tilt y34 in the expression (4) isminimized.

Also in the optical transmission apparatus 10A-3, the excitation lightfrequency νp1 of the wavelength converter 131A and the excitation lightfrequency νp4 of the wavelength converter 132A are calculated accordingto the same procedure using the table information received from theoptical transmission apparatus 10B-3. In this case, the values ofνp1=νp2 and νp4=νp3 are calculated, and the frequencies νp1 and νp4 areset to the pump light sources 133A and 134A respectively.

FIG. 18 is a diagram for explaining the effect of setting the excitationlight frequency. In the third embodiment, since for the wavelengthconversion characteristic, actual measurement value data is used withoutperforming fitting, the characteristic curve of the HNLF1 and thecharacteristic curve of the HNLF4 are distorted. The solid curveindicates the characteristic of the totaled tilt. The frequency at whichthe total amount of tilt is minimized is selected as the excitationlight frequency νp2 of the pump light source 134B. In the wavelengthconverter 131B, the excitation light frequency νp3 at which the totalamount of tilt is minimized is selected and set. In the opticaltransmission apparatus 10A-3, the same excitation light wavelength asthat of the optical transmission apparatus 10B-3 is set by the sameprocedure.

The method of FIG. 18 is used effectively when the error of quadraticfunction approximation is large, or when the difference between the zerodispersion frequencies of the HNLF1 and the HNLF2 is large. In thismethod, an excitation light frequency that is the same excitation lightfrequency between the optical transmission apparatus 10A-3 and theoptical transmission apparatus 10B-3 and at which the tilt of the mainsignal light that has undergone the wavelength conversion twice betweenthe optical transmission apparatus 10A-3 and the optical transmissionapparatus 10B-3 is minimized is autonomously set, and it is possible toreceive the main signal light with uniform wavelength conversionefficiency within the ITU grid.

In the optical transmission system 1-3 according to the thirdembodiment, the excitation light frequency is recalculated at the timeof replacement of the failed supervisory control light transmitting unit141 or the failed supervisory control light receiving unit 142 inaddition to when the wavelength converter 131 or 132 is replaced, andwhen a new optical transmission apparatus 10 is introduced. Theexcitation light frequency is recalculated at the time of recovery fromthe interruption of the OSC signal, and at the time of recovery fromdisconnection of the optical transmission line 2 or 3. The fundamentalflow of the wavelength conversion and transmission/reception of theoptical signal after setting the excitation light frequency is the sameas that of the optical transmission system 1-1 according to the firstembodiment.

Fourth Embodiment

FIG. 19 is a schematic diagram of an optical transmission system 1-4according to the fourth embodiment. The optical transmission system 1-4includes an optical transmission apparatus 10A-4 (A station), and anoptical transmission apparatus 10B-4 (B station). The basicconfiguration of the optical transmission system 1-4 is the same as thebasic configuration of the optical transmission system 1-2 according tothe second embodiment. The features according to the fourth embodimentwill be mainly described.

In the fourth embodiment, a supervisory control unit 146 connected to asupervisory control network 41 and a network element operation system(NE-OPS) 42 is used as the HNLF characteristic communication interfaces14 and 15. The supervisory control unit 146 is implemented by, forexample, a CPU and a network interface. Information on the wavelengthconversion characteristic is transmitted and received between theoptical transmission apparatus 10A-4 and the optical transmissionapparatus 10B-4 by the NE-OPS 42.

In the example of FIG. 19, the optical transmission apparatus 10A-4 andthe optical transmission apparatus 10B-4 hold a set of coefficients (a,b, c) of a quadratic approximate curve as the wavelength conversioncharacteristic, but they are not limited to this example. As in thefirst embodiment, the zero dispersion frequency of the wavelengthconverters 131 and 132 may be held. As in the third embodiment, the HNLFtable may be held.

The NE-OPS 42 reads requested wavelength conversion characteristicvalues via the supervisory control network 41 to transfer it to a targetoptical transmission apparatus 10-4. The procedure of transferring thewavelength conversion characteristic information from the opticaltransmission apparatus 10A-4 to the optical transmission apparatus 10B-4is as follows.

(1) The NE-OPS 42 transmits a request for reading the HNLFcharacteristic to a supervisory control unit 146A of the opticaltransmission apparatus 10A-4.

(2) The supervisory control unit 146A transfers the request for readingthe HNLF characteristic to the wavelength conversion unit 13A.

(3) The wavelength conversion unit 13A reads the HNLF characteristicvalue from the HNLF characteristic holding memory 136A, and supplies theread value to the supervisory control unit 146A.

(4) The supervisory control unit 146A of the optical transmissionapparatus 10A-4 transmits the HNLF characteristic value to the NE-OPS42.

(5) The NE-OPS 42 transmits the HNLF characteristic value received fromthe optical transmission apparatus 10A-4 to a supervisory control unit146B of the optical transmission apparatus 10B-4.

(6) The supervisory control unit 146B supplies the received HNLFcharacteristic value of the optical transmission apparatus 10A-4 to theexcitation light frequency control unit 135B.

(7) The excitation light frequency control unit 135B calculates and setsthe excitation light frequencies νp2 and νp3 at which the total amountof tilt is minimized between the opposed optical transmissionapparatuses using the HNLF characteristic value of the opticaltransmission apparatus 10A-4 and the HNLF characteristic value stored inthe HNLF characteristic holding memory 136B of the own station.

Transfer of the HNLF characteristic value from the optical transmissionapparatus 10B-4 to the optical transmission apparatus 10A-4, andcalculation of the excitation light frequencies νp1 and νp4 in theoptical transmission apparatus 10A-4 are also performed in the samemanner. For HNLF characteristic information, when zero dispersionfrequency or table information is transferred, the excitation lightwavelength at which the tilt of the main signal light having undergonethe wavelength conversion twice is minimized is calculated by the methoddescribed in the first embodiment and the third embodiment.

In the optical transmission system 1-4 according to the fourthembodiment, the excitation light frequency is recalculated when theoptical transmission apparatus 10 performing the wavelength conversionis not monitored and thereafter restored, and when the wavelengthconversion unit 13 is not monitored and thereafter restored in additionto when the wavelength converter 131 or 132 is replaced, and when a newoptical transmission apparatus 10 is introduced. “is not monitored”means that the control operation of the optical transmission apparatus10 is not seen from the NE-OPS 42 due to a failure of the supervisorycontrol unit 146, the control operation of some groups is invisible fromthe NE-OPS 42 when switching the groups on which wavelength multiplexingof signals of a plurality of transponders are performed to perform thewavelength conversion, or the like.

The excitation light frequency is recalculated at the time of recoveryfrom disconnection of the optical transmission line 2 or 3. Thefundamental flow of the wavelength conversion and transmission/receptionof the optical signal after setting the excitation light frequency isthe same as that of the optical transmission system 1-1 according to thefirst embodiment.

Fifth Embodiment

FIG. 20 is a diagram for explaining a method of setting an optimumexcitation light frequency according to a fifth embodiment. In the firstto fourth embodiments, the excitation light frequency between theoptical transmission apparatus 10A and the optical transmissionapparatus 10B on the reception side is set to be the same.

In practical communication, even if the excitation light frequency onthe transmission side and the excitation light frequency on thereception side do not completely coincide with each other, it ispossible to receive the WDM signal in the ITU grid as long as thefrequency difference is within the allowable range, and the loss at thetime of demultiplexing is small.

The left diagram of FIG. 20 illustrates a schematic diagram when theexcitation light frequencies on the transmission side and the receptionside coincide with each other. The right diagram of FIG. 20 illustratesa schematic diagram when excitation light frequencies on thetransmission side and the reception side are different from each other.

In the left figure, the excitation light frequency νp1 on thetransmission side (for example, the optical transmission apparatus 10A)and the excitation light frequency νp2 on the reception side (forexample, the optical transmission apparatus 10B) coincide with eachother. In this case, when performing the second-time collectiveconversion (for example, conversion from the L band to the C band) onthe reception side is performed after the first-time collectiveconversion (for example, conversion from the C band to the L band) onthe transmission side is performed, it is possible to return thewavelength after conversion to substantially the same wavelength as thewavelength before conversion. Loss of light hardly occurs in eachwavelength signal extracted by the DEMUX filter on the reception side.

In the right figure, when the excitation light frequency νp1 of thetransmission side and the excitation light frequency νp2 on thereception side are deviated, excessive loss occurs in each wavelengthsignal extracted by the DEMUX filter after the second-time collectiveconversion. This light loss is caused by the frequency deviation betweenνp1 and νp2.

When the frequency deviation between νp1 and νp2 is within an allowablerange, the offset of the wavelength signal extracted after thesecond-time conversion from the original signal is small, wherebysufficient optical power may be obtained.

FIG. 21 is a diagram for explaining the setting of an excitation lightwavelength with a frequency difference within an allowable range. Whenthe horizontal axis represents the excitation light frequency on thetransmission side and the vertical axis represents the excitation lightfrequency on the reception side, the solid line extending in thedirection of 45° from the origin represents the line indicating that thefrequency difference is zero, for example, the line at which theexcitation light frequencies on the transmission side and the receptionside coincide with each other.

The gray area including the solid line is an area where the frequencydifference is within the allowable range. Within the gray area, even ifthe excitation light frequencies on the transmission side and thereception side slightly deviates, the loss of the optical power is small(within the allowable range), whereby it is possible to correctlydemodulate each received wavelength signal. Therefore, in the gray area,the excitation light frequency at which the total amount of tilt isminimized is searched for.

FIG. 22 is a diagram for explaining the setting of an optimum excitationlight frequency with a frequency difference within an allowable range.By the method of the second embodiment (acquiring the coefficientinformation of the approximate curve) or the method of the thirdembodiment (acquiring the table information), the amount of tilts of thetransmission side and the reception side is acquired, and the totalamount of tilt for each excitation light frequency is calculated.

In FIG. 22, the leftmost column represents the excitation lightfrequency on the transmission side, and the uppermost row represents theexcitation light frequency on the reception side. A cell where twoexcitation light frequencies intersect represents the total amount oftilt at the excitation light frequency. For reference, the amount oftilt occurring on the reception side is illustrated outside of the rightcolumn. The amount of tilt occurring on the transmission side isillustrated outside of the lower row. The numbers in the table are thesum of the amount of tilt on the reception side and the amount of tilton the transmission side.

In FIG. 22, an unshaded white area is an area where the frequencydifference is within an allowable range. In this white area, acombination at which the total amount of tilt is minimized is selected.In the example of FIG. 22, when the excitation light frequency on thetransmission side is 191.32 THz, and the excitation light frequency ofthe reception side is 191.30 THz, the total amount of tilt is minimized.

The selected excitation light frequency is set on the transmission sideand the reception side. The calculation of FIG. 22 is performed for eachof the optical transmission apparatus 10A and the optical transmissionapparatus 10B after the wavelength conversion characteristic informationis exchanged between the optical transmission apparatus 10A on thetransmission side and the optical transmission apparatus 10B on thereception side. For example, an excitation light frequency control unit133 of the wavelength conversion unit 13 of each optical transmissionapparatus 10 performs the calculation (see FIGS. 10 and 15). Thewavelength conversion unit 13 of each optical transmission apparatus 10may select the excitation light frequency of the own station at whichthe total amount of tilt is minimized and may also recognize theexcitation light frequency of the partner station.

By setting the allowable range of the frequency difference, a smallamount of tilt may be achieved, compared with the case where theexcitation light frequency on the transmission side and the receptionside which coincide with each other is selected. Cells where theexcitation light frequencies on the transmission side and the receptionside coincide with each other are arranged on an oblique line from theupper left to the lower right in FIG. 22. The minimum amount of tilt onthis line is the value “1” when the excitation light frequency is 191.31THz on both the transmission side and the reception side.

On the other hand, the minimum amount of tilt selected within theallowable range is “0.3”. The wavelength conversion is implemented witha smaller amount of tilt. In the fifth embodiment, a predeterminedallowable range is set with respect to a difference between excitationlight frequencies on the reception side and the transmission side isset, whereby it is possible to autonomously set a combination of optimumexcitation light frequencies at which the total amount of tilt isminimized.

Modification

FIGS. 23A and 23B are schematic diagrams of an optical transmissionsystem 1-5 according to a modification. The optical transmission system1 according to the embodiment is also applied to an optical network inwhich the optical transmission apparatus 10 are connected with eachother via a relay station.

The optical transmission system 1-5 includes an optical transmissionapparatus 10A-5 (A station), an optical transmission apparatus 10B-5 (Bstation), and a relay station 50 (C station) that relays between them.The optical transmission apparatus 10A-5 and the relay station 50 areconnected with each other through optical transmission lines 2 a and 3b. The relay station 50 and the optical transmission apparatus 10B-5 areconnected with each other through optical transmission lines 2 b and 3a.

The HNLF characteristic information stored in the HNLF characteristicholding memory 136 of each optical transmission apparatus 10 istransmitted and received by the supervisory control light transmittingunit 141 and the supervisory control light receiving unit 142, which areHNLF characteristic communication interfaces, using the OSC.

When transferring HNLF characteristic with supervisory control light,the relay station 50, which does not perform the wavelength conversion,has a unit that transfers HNLF characteristic information to thenext-stage optical transmission apparatus 10.

The relay station 50 includes supervisory control light transmittingunits 51 and 55, and supervisory control light receiving units 52 and56. When HNLF characteristic information is transmitted from the opticaltransmission apparatus 10A-5 to the optical transmission apparatus10B-5, the relay station 50 receives the HNLF characteristic informationat the supervisory control light receiving unit 52, and transfers theHNLF characteristic information at the supervisory control lighttransmitting unit 51. When HNLF characteristic information istransmitted from the optical transmission apparatus 10B-5 to the opticaltransmission apparatus 10A-5, the relay station 50 receives the HNLFcharacteristic information at the supervisory control light receivingunit 56, and transfers the HNLF characteristic information at asupervisory control light transmitting unit 55.

The supervisory control light transmitting units 51 and 55, and thesupervisory control light receiving units 52 and 56 may be implementedusing one OSC card or one OSC module.

The client signal is wavelength-converted from the signal light ν1 tothe signal light ν2 by the optical transmission apparatus 10A-5, and isoutput to the optical transmission line 2 a. The signal light ν2 isamplified by an optical amplifier 53 of the relay station 50 andtransferred to the optical transmission apparatus 10B-5. The opticaltransmission apparatus 10B-5 wavelength-converts the received signallight ν2 into the signal light ν3 of the original wavelength, andoutputs it to the transponder 11B.

The client signal is wavelength-converted from the signal light ν4 tothe signal light ν5 by the optical transmission apparatus 10B-5, and isoutput to the optical transmission line 3 a. The signal light ν5 isamplified by an optical amplifier 54 of the relay station 50, andtransferred to the optical transmission apparatus 10A-5. The opticaltransmission apparatus 10A-5 wavelength-converts the received signallight ν5 into the signal light ν6 of the original wavelength, andoutputs it to the transponder 11A.

In the optical transmission system 1-5 in FIG. 23A and 23B, theexcitation light frequency is recalculated at the time of recovery fromthe optical transmission line disconnection in the relay section inaddition to when the wavelength converter 131 or 132 is replaced, andwhen a new optical transmission apparatus 10 is introduced. Theexcitation light frequency is recalculated at the time of replacement ofthe failed supervisory control light transmitting unit 141 or the failedsupervisory control light receiving unit 142, and at the time ofrecovery from the interruption of the OSC signal.

The configuration of FIG. 23 is effectively used, for example, forlong-distance optical communication.

FIGS. 24A and 24B illustrate an optical transmission system 1-6 asanother modification. In the optical transmission system 1-6, transferof the main signal light using an relay station 60, and transfer of HNLFcharacteristic information using NE-OPS 42 are combined. In thisexample, the relay station 60 may not transfer the HNLF characteristicinformation, and the main signal light is amplified and transferred bythe optical amplifiers 63 and 64.

The transmission and reception of the HNLF characteristic informationbetween an optical transmission apparatus 10A-6 and an opticaltransmission apparatus 10B-6 is performed according to the proceduredescribed in the fourth embodiment. The information held in the HNLFcharacteristic holding memory 136 of the optical transmission apparatus10-6 may be the zero dispersion wavelength of the wavelength converters131 and 132, a coefficient of an approximate curve representing thewavelength conversion characteristic, or a table in which the excitationfrequency and the amount of tilt are associated with each other.

In any configuration example, the characteristic information of thewavelength converter is exchanged between the adjacent opticaltransmission apparatuses 10, and the excitation light frequency at whichthe gain deviation of the main signal light after performing thewavelength conversion twice is minimized is calculated using wavelengthconversion characteristic information of the own station and wavelengthconversion characteristic information of the partner station.

Although the present embodiments have been described using specificconfiguration examples, the present embodiments are not limited to theabove-described examples, and includes various modifications withoutdeparting from the subject matter of the embodiments. Among theabove-described embodiments, two or more embodiments may be combined.For example, zero dispersion frequency or table information may beacquired using the HNLF characteristic acquisition method used in thesecond embodiment in the first embodiment and the third embodiment.

The wavelength conversion configuration according to the embodiment mayalso be applied to an optical relay node. For example, in theconfiguration of FIGS. 23A and 23B, the wavelength conversion unit 13 isprovided in the relay station 50, and HNLF characteristic informationmay be exchanged between the relay station 50 and the opticaltransmission apparatus 10A-5, and between the relay station 50 and theoptical transmission apparatus 10B-5 to set the optimum excitation lightfrequency. Collective wavelength conversions at the optical relay nodemake it possible to avoid collision of wavelength.

The wavelength converter is not limited to a wavelength converter usingHNLF, but may be a wavelength converter using a nonlinear opticalcrystal such as LiNbO3 (lithium niobate). Also in this case, theconversion characteristic of the nonlinear optical crystal (refractiveindex characteristic or dispersion characteristic) is measured inadvance, and stored a memory. Use of the conversion characteristictogether with conversion characteristic information received from thepartner station makes it possible to set the frequency of the excitationlight to the optimum frequency.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. An optical transmission apparatus comprising: awavelength converter that wavelength-converts input signal light using anonlinear optical medium to output the converted signal light; a memorythat holds first information relating to a wavelength conversioncharacteristic of the wavelength converter; a communication interfacethat receives second information relating to a second wavelengthconversion characteristic of an adjacent optical transmission apparatus;and a control circuit that determines, using the first information andthe second information when the second information is received, anexcitation light frequency at which a gain deviation of main signallight subjected to a wavelength conversion is minimized to set thedetermined excitation light frequency in the wavelength converter. 2.The optical transmission apparatus according to claim 1, wherein thecommunication interface notifies the adjacent optical transmissionapparatus of the first information.
 3. The optical transmissionapparatus according to claim 1, wherein the memory stores a first zerodispersion frequency of the nonlinear optical medium as the firstinformation, wherein the communication interface receives, as the secondinformation, a second zero dispersion frequency of a second wavelengthconverter used in the adjacent optical transmission apparatus, andwherein the control circuit determines as the excitation light frequencyan average value of the first zero dispersion frequency and the secondzero dispersion frequency.
 4. The optical transmission apparatusaccording to claim 1, wherein the memory stores, as the firstinformation, first approximate curve information indicating thewavelength conversion characteristic of the wavelength converter as afunction of an excitation frequency, wherein the communication interfacereceives, as the second information, second approximate curveinformation indicating the second wavelength conversion characteristicas a function of an excitation frequency, and wherein the controlcircuit adds the first approximate curve information to the secondapproximate curve information to determine, as the excitation lightfrequency, a frequency at which an addition approximate curve isminimized.
 5. The optical transmission apparatus according to claim 4,wherein the approximate curve information is a coefficient of aquadratic function indicating the approximate curve.
 6. The opticaltransmission apparatus according to claim 1, wherein the memory stores,as the first information, a first table in which an amount ofdeterioration of a conversion efficiency of the wavelength converter isdescribed in association with a first excitation frequency, wherein thecommunication interface receives, as the second information, a secondtable describing an amount of deterioration of a conversion efficiencyof a second wavelength converter of the adjacent optical transmissionapparatus in association with a second excitation frequency, and whereinthe control circuit adds a value of the first table with respect to thefirst excitation frequency to a value of the second table with respectto the second excitation frequency to determine, as the excitation lightfrequency, a frequency at which a total amount of deterioration isminimized.
 7. The optical transmission apparatus according to claim 6,wherein the control circuit sets, with respect to the excitation lightfrequency, an allowable offset range in which the optical transmissionapparatus is capable of receiving the main signal light to determine, asthe excitation light frequency, a frequency at which the total amount ofdeterioration is minimized within the allowable offset range.
 8. Theoptical transmission apparatus according to claim 1, wherein thecommunication interface is an optical supervisory module or an opticalsupervisory card that transmits and receives a supervisory controlsignal using an optical supervisory channel.
 9. The optical transmissionapparatus according to claim 1, wherein the communication interface is asupervisory control module coupled to a supervisory control network, andwherein the optical transmission apparatus acquires the secondinformation from the supervisory control network.
 10. A method ofcontrolling an excitation light frequency, the method comprising: in anoptical transmission apparatus including a wavelength converter using anonlinear optical medium, the optical transmission apparatuswavelength-converting input signal light to output the converted signallight, storing first information relating to a wavelength conversioncharacteristic of the wavelength converter in a memory in advance; whensecond information relating to a second wavelength conversioncharacteristic of an adjacent optical transmission apparatus is receivedby the optical transmission apparatus, determining, using the firstinformation and the second information, an excitation light frequency atwhich a gain deviation of main signal light subjected to a wavelengthconversion is minimized; and setting the determined excitation lightfrequency in the wavelength converter.